1. Field of the Invention
The present invention relates to a planetary roller type flow control valve, which is available in an apparatus requiring extremely precise control for a minute flow rate such as control for a fuel flow rate and the like.
2. Description of the Prior Art
As representative flow control valves in the prior art, a needle valve type flow control valve as shown in FIG. 12, a slitted cylinder type flow control valve as shown in FIG. 13, and a slitted disc type flow control valve as shown in FIG. 14 are known. In the needle valve type flow control valve, an opening area of a fluid outlet 51 is varied by a needle valve 52 entering and coming out in the axial direction with respect to the fluid outlet 51 as shown in FIG. 12 and thereby a flow rate is controlled, and the entering and coming out of the needle valve 52 is effected by rotationally driving the needle valve 52 with a stepping motor 57. The needle valve 52 has its outer circumference tightly sealed by a seal ring 54 so that fluid entering through a fluid inlet 53 may not leak out in the opposite direction. It is to be noted that reference numeral 56 denotes a coupling member between the needle valve 52 and the stepping motor 57.
In the slitted cylinder type flow control valve, a flow rate is controlled by a notch or hole 34 formed in a shaft of a stepping motor 36 opening and closing a slit 33 drilled on the side of a fluid inlet 32 according to rotation of the shaft as shown in FIG. 13. It is to be noted that in this figure reference numeral 31 denotes a fluid outlet and numeral 35 denotes a seal member.
In the slitted disc type flow control valve, a flow rate is controlled by a notch or hole 44 in a disc 49, which is rotatably held in contact with a wall provided in perpendicular to a fluid outlet 41 within a chamber between a fluid inlet 42 and the fluid outlet 41, opening and closing a slit 43 in the same wall according to rotation of the disc 49 as shown in FIG. 14. In this figure, reference numeral 47 denotes reduction gears for transmitting rotation of a stepping motor 46 to the disc 49 while appropriately reducing the rotational speed, numeral 48 denotes a preloading spring for making a pressing force of the disc 49 against the wall sufficient, and numeral 45 denotes a seal member.
The above-described flow control valves (hereinafter called simply "valve") in the prior art involved the following problems to be resolved.
That is, in order to precisely control a minute flow rate, it is necessary to exactly regulate a throttle area of 0.1 mm.sup.2 or less. However, in the flow control valve of needle valve type in the prior art, despite of the fact that a needle valve having a diameter and a stroke of 0.4 mm or less and a driving device (a combination of a ball screw and a stepping motor) were necessitated, there was a problem that due to machining errors a sufficient precision could not be obtained, and also there was a problem that upon full closure a valve seat for a needle would wear and hence its durability was poor. In the slitted type valve and slitted disc type valves in the prior art, a slit of about 0.02 mm.times.5 mm was necessary, hence there was a problem that a possibility of foreign matters in fluid such as fuel or the like blocking the slit was large, also despite of the fact that in order to realize full closure it is necessary to make a gap clearance between slide surfaces zero, in the slitted cylinder type valve it was impossible to be realized, in the slitted disc type valve it was necessary to hold slide surfaces in press contact by means of a spring or the like, and there was a problem that friction upon driving would become large.
Furthermore, in the case where the needle valve or the slitted cylinder type and slitted disc type valves in the prior art are driven by a stepping motor controlled by a digital controller, it is necessary to enlarge a driving force and also enhance a control resolution by making use of a speed reducing device (a ball screw, a gear-box or the like). However, since a driving torque of a stepping motor used for control purpose is normally as small as several kg.multidot.cm, in order to fulfil this requirement it is necessary to use a speed reducing device having a sufficiently large reduction ratio so as to overcome a frictional force of a needle valve or a slit valve. But, if large speed reduction is effected, a response characteristic is degraded, and so, there is a problem that an aimed control function cannot be achieved.